This invention relates to an encoder for encoding a multilevel or multiple-level picture element (usually abbreviated to "pel") signal sequence into a compression encoded signal, a decoder therefor, and an encoder-decoder system comprising such an encoder and a decoder.
An encoder of the above-specified type remarkably reduces the amount of information or data to be either transmitted or stored. It is thereby possible to raise the speed of transmission and to reduce the memory capacity of a memory in which the multilevel picture element signal sequence is to be stored.
Predictive encoding is effective in carrying out the data compression encoding. According to the predictive encoding technique, an actual level of each picture element signal in a multilevel picture element signal sequence is predicted by the use of a predetermined number of picture element signals neighboring the picture element signal under consideration among the picture element signals preceding that picture element signal in the sequence. Encoding is carried out for a sequence of error signals, each representative of the difference between the actual level and the predicted level of the picture element signal in question. The accuracy of prediction is rendered high to raise the data compression efficiency when nonlinear prediction is resorted to. A large capacity memory is, however, indispensable in producing the error signals as will later be discussed particularly when the multilevel picture element signal sequence is variable among a great number of levels. Therefore, the encoder becomes accordingly bulky. This applies to a color picture element signal sequence in which the difference among colors is represented by a great number of levels.
In facsimile transmission of a two-level picture element signal sequence, one-dimensional run length encoding is widely resorted to. The above-described error signals are, however, not conveniently encoded by the run length encoding technique as will soon be described with reference to a few of about twenty figures of the accompanying drawing when the number of levels is more than two.
Furthermore, a known mode run length encoding technique applied to a color picture element signal sequence is not effective in carrying out the data compression. According to the technique, a duration of a particular color during a certain time interval is considered as a run of a run length corresponding to the duration. A run length code is generated to represent the run length, together with a mode code that specifies the particular color. The frequency of appearance of a specific color is taken into consideration on deciding the run length codes and the mode codes. The correlation between colors is, however, entirely neglected. For example, the run length codes and the mode codes are predetermined without any consideration on the fact that either orange or green appears more frequently than other colors, the next most frequent color being yellow.
Accordingly, a large capacity memory becomes indispensable in expansion decoding the compression encoded signal derived by nonlinear prediction from a multilevel picture element signal sequence variable among many levels. This applies to both a monochromatic and a color picture element signal sequence.